Heretofore, generally, in addition to iron-based materials, copper-based materials, such as phosphor bronze, red brass, and brass, which are excellent in electrical conductivity and thermal conductivity, have been used widely as materials for electric and electronic equipments (electrical and electronic machinery and tools). Recently, demands for miniaturization, lightening of the weight, high-functionalization, and associated high-density packaging of parts of electric and electronic equipments have increased, and various characteristics of higher levels are required for the copper-based materials applied thereto.
For example, a copper alloy to be used for a CPU socket and the like is required to have higher electrical conductivity than conventional copper alloys for heat removal, according to the increase of heat emission of CPU. Further, the environment of use of dedicated automobile-mounted connectors has become severe, and higher electrical conductivity is required for the copper alloy for the terminal materials in order to improve heat radiation.
Meanwhile, thinning of the material has been advanced in association with miniaturization of the parts, with a requirement of improvement of the strength of the material. Requirement of fatigue resistance is also enhanced in the application of relays and the like, with improvement of the strength. Since the conditions for bending the material have become severe in association with miniaturization of the parts, it is required for the material to be excellent in bending property while high strength is maintained. Further, better dimensional accuracy of the parts are also required in association with miniaturization of the parts, and the amount of displacement of spring material at a portion for releasing contact pressure has become lessened. Since permanent setting of the material under a long term use becomes an issue as compared to before, the material is required to have high resistance to stress relaxation. The requirement for stress relaxation resistance is further enhanced, for example, in automobiles, since the environmental temperature for use is high.
These required characteristics have reached to a level that cannot be satisfied with commercially available mass-production alloys, such as phosphor bronze, red brass, and brass. These alloys are enhanced in the strength, by forming a solid solution of tin (Sn) and zinc (Zn) in copper (Cu), followed by subjecting the alloy to cold-working, such as rolling and drawing. However, it is known that while a high strength material may be obtained by applying a high cold-working ratio (generally, 50% or more) by this method, bending property of the resultant alloy is conspicuously impaired, in addition to poor electrical conductivity. This method is generally a combination of solid solution hardening and work hardening.
An alternative of the hardening method is a precipitation hardening method by which the material is hardened by forming nanometer-ordered fine precipitates in the material. This method is applied to many alloy systems, since this method enhances the strength while it has an advantage for simultaneously improving electrical conductivity. Among many precipitation-type alloys, a so-called Corson alloy which is hardened, by adding nickel (Ni) and silicon (Si) in Cu, and by allowing Ni—Si compounds to finely precipitate, has a quite high hardening ability, and is used in some commercially available alloys (for example, CDA 70250 that is a registered alloy of CDA (Copper Development Association)).
Generally, the following two important heat-treatment steps are used in the production process of precipitation hardened-type alloys. One is a heat treatment, called as a solution treatment, for allowing Ni and Si to dissolving into a Cu matrix at a high temperature (generally, 700° C. or higher); and the other is a so-called aging precipitation, which is a heat treatment to be conducted at a lower temperature than the temperature for the solution treatment. The latter is applied for allowing Ni and Si dissolved at a high temperature to precipitate as a precipitate. This hardening method takes advantage of a difference of the amounts of Ni and Si atoms that are dissolved in Cu between at a higher temperature and at a lower temperature, and is well known in the art in the method for producing precipitation-type alloys.
Although the amount of use of the Corson-system alloy is increasing, electrical conductivity of the alloy is not sufficient against high required characteristics as described above. Meanwhile, a Cu—Ni—Co—Si-based alloy in which a part of Ni in the Corson-based alloy is substituted with cobalt (Co) is known (for example, JP-T-2005-532477 (“JP-T” means published searched patent publication)). This alloy system includes precipitation hardened-type alloys of compounds, such as Ni—Co—Si, Ni—Si, and Co—Si, and is featured in smaller solid solution limit than the Corson-based alloy. This alloy system is advantageous in realizing high electrical conductivity since the amount of elements in the solid solution is small.
Contrary to the advantage, the solution treatment temperature is required to be higher than the corresponding temperature in the Cu—Ni—Si system, due to a small solid solution limit. Since the amount of the dissolved element(s) becomes small upon the solution treatment when the solution temperature cannot be raised up, the magnitude of precipitation hardening becomes low in the aging precipitation heat treatment, and the strength is required to be compensated by work hardening at a relatively high working ratio. Consequently, there arises such a problem that bending property that is an important characteristic required may be impaired, due to coarsening of crystal grains when the solution heat treatment temperature is high, or due to increase of dislocation density in the material when work hardening at a relatively high working ratio is introduced. Therefore, these treatments are not able to satisfy the required characteristics of the copper material that are enhanced in the fields of electronic equipments and automobiles in recent years.
For controlling bending property in Cu—Ni—Si alloy systems, accumulation of crystal orientation has been prescribed by X-ray diffraction intensity of the surface of the alloy sheet (for example, Japanese Patent No. 3,739,214). However, this invention relates to a method for controlling the crystal grain diameter by adjusting the conditions for solution heat treatment and for reducing the amount of work hardening, and is not suitable for the above-mentioned requirement of solution heat treatment at a high temperature as in Cu—Ni—Co—Si alloys, since the treatment causes deterioration in the strength and bending property.